1. Field of the Invention
The present invention relates generally to gas separation membranes and, more particularly, to a metal membrane for the separation of hydrogen gas from a gas stream.
2. Description of the Related Art
Membranes and membrane modules for the separation of hydrogen from other gases are generally known. In particular, useful membranes for hydrogen separation typically can be categorized as being of four general types: (i) polymeric, (ii) porous inorganic, (iii) self-supporting non-porous metal, and (iv) non-porous metal supported on a porous rigid matrix such as metal or ceramic.
Polymeric membranes are commonly used in the form of extended flat sheets of small diameter hollow fibers. Flat sheet polymeric membranes are most often incorporated into spiral-wound modules. Hollow fiber membranes are incorporated into hollow-fiber modules which are very similar in design to shell-and-tube heat exchangers.
Polymeric membranes and membrane modules for hydrogen separation suffer from a lack of high selectivity toward hydrogen over other gases, which results in a relatively impure product gas. Such membranes also suffer from a lack of stability at operating temperatures above 480° F. (250° C.) and chemical incompatibility with many chemicals such as hydrocarbons that are present in an impure hydrogen feed stream.
Porous inorganic-based membranes are typically fabricated from titania, zirconia, alumina, glass, molecular sieving carbon, silica and/or zeolites. All are fabricated with a narrow pore size distribution, with the porous inorganic membranes exhibiting high hydrogen permeability but low selectivity due to relatively large mean pore diameters. Such materials are brittle and thus susceptible to failure due to cracking, and the sealing and fixturing of such porous inorganic-based membranes limit their use to relatively low temperature applications.
Development of supported metal membranes has focused on the utilization of ceramic tubes coated with a thin film or foil of non-porous or dense palladium (Pd) or Pd-alloys. The ceramic support tube typically is of a graded porosity from one surface thereof to a second opposite surface. More specifically, the porosity of the ceramic support tube typically is at its least at the surface upon which the Pd or Pd-alloy is disposed, and the porosity of the tube increases from this surface to a maximum porosity on the surface opposite the layer of Pd. The layer of Pd or Pd-alloy is selectively permeable to hydrogen gas and is typically capable of withstanding temperatures of 1500-1600° F. (815-870° C.).
Such ceramic-supported metal membranes are typically housed in shell and tube modules and are fitted with compression gaskets to seal the membrane tube into the module to prevent leakage of the feed gas stream into the permeate gas stream. Potential leak paths between the feed and permeate gas streams can exist due to differences in the coefficients of thermal expansion of the ceramic tube and the metal compression fittings. Additionally, the ceramic support tubes are inherently brittle and can experience long-term thermal fatigue due to repetitive process or system start-up and shutdown cycles.
The mechanical adherence of the thin Pd or Pd-alloy layer upon the surface of the ceramic support tube requires secure attachment of the film or foil onto the surface of the ceramic, as well as the absence of pinholes or other mechanical rupturing that can occur during manufacture or use of the ceramic tube membrane.
For porous metal membranes such as porous stainless steel and microporous noble metals, Knudsen diffusion or combined Knudsen diffusion/surface diffusion are the primary mechanisms by which gas transport occurs across the membrane. For dense metal membranes such as Pd or Pd-alloy foil or film, however, the primary mechanism of gas transport through the metal layer is traditional chemisorption-dissociation-diffusion. Broadly stated, chemisorption-dissociation-diffusion transport involves chemisorption of hydrogen molecules onto the membrane surface, dissociation of hydrogen atoms into protons and electrons, transportation of the protons and electrons through the dense metal, reassociation of the protons and electrons into hydrogen molecules, and desorption of hydrogen molecules from the media. While Knudsen diffusion typically offers greater flow rates across a membrane than chemisorption-dissociation-diffusion, Knudsen diffusion suffers from reduced hydrogen selectivity as compared with chemisorption-dissociation-diffusion.
It is also known that the interaction of a gas stream with catalytic materials can increase the concentration of hydrogen within the reactant or process gas stream. Such catalytic materials enhance the water gas shift reaction whereby carbon monoxide is reacted with water to form carbon dioxide and hydrogen gas. Catalytic materials also promote the decomposition of ammonia which also increases the concentration of hydrogen.
Examples of such catalytic materials include platinum (Pt), palladium, rhodium (Rh) and the like. While it has been known to apply such catalytic materials to ceramic support substrates to form composite membranes, such composite membranes still suffer from the aforementioned problems associated with the application of Pd and Pd-alloy foils and films to ceramic support tubes.
It is thus desired to provide a hydrogen gas separation membrane having high hydrogen selectivity that overcomes the aforementioned problems associated with the application of Pd and Pd-alloys to ceramic support tubes. It is also desired to provide such a membrane that includes a catalytic enhancement incorporated therein.